This invention relates generally to an encoder, and more particularly, to an encoder for a sinusoidal brushless motor of an Electric Power Steering (EPS) system.
Electric power steering (EPS) has been the subject of development by auto manufacturers and suppliers for over a decade because of its fuel economy and ease-of-control advantages compared with the traditional hydraulic power steering (HPS). However, commercialization of EPS systems has been slow and is presently limited to small and midget-class cars due to cost and performance challenges. Among the most challenging technical issues are the pulsating feel at the steering wheel and the audible noise associated with the type of high performance electric drives needed to meet the steering requirements.
The choice of motor type for an EPS determines the characteristics of the drive and the requirements relating to power switching devices, controls, and consequently cost. Leading contenders are the Permanent Magnet (PM) brushless motor, the Permanent Magnet (PM) commutator-type and the switched reluctance (SR) motors; each of the three options has its own inherent advantages and limitations. The PM brushless motor is based upon years of experimenting with commutator-type motors. The large motor size and rotor inertia of commutator-type motors limit their applicability to very small cars with reduced steering assist requirements. Additionally, the potential for brush breakage that may result in a rotor lock necessitates the use of a clutch to disconnect the motor from the drive shaft in case of brush failure. SR drives offer an attractive, robust and low cost option, but suffer from inherent excessive torque pulsation and audible noise, unless special measures are taken to reduce such effects. For column assist applications, the motor is located within the passenger compartment and therefore must meet stringent packaging and audible noise requirements that the present SR motor technology may not satisfy. Therefore, the PM brushless motor with its superior characteristics of low inertia, high efficiency and torque density, compared to commutator motors, appears to have the potential for not only meeting the present requirements but also those of future high performance EPS systems of medium and large vehicles.
Despite the relatively low levels of torque ripple and noise of EPS systems using conventional PM brushless motors, they are no match to the smoothness and quietness of HPS with a decades-long history of performance refinement efforts. Consumers are reluctant in compromising such features. Therefore, a new torque ripple free (TRF) system is needed, which, as the name indicates, would eradicate the sources of torque ripple (under ideal conditions) and reduce noise levels considerably. The near term goal is to enhance the performance of EPS systems with the long-term objective of increasing acceptability of EPS systems for broader usage.
Several performance and cost issues have stood in the way of broad-based EPS commercialization regardless of the technology used, but with varying degree of difficulty. In order to generate motor currents with a sinusoidal shape, the inverter switching devices (e.g. MOSFETS) must be turned on and off at specific rotor angular positions. Therefore, the position of the rotor must be known at all times and an encoder is needed. This requirement is one of the factors adding to the cost of sinusoidal drives, hence traditionally limiting their application to high-performance applications. EPS is a high-performance drive, however it must meet stringent cost limits. Therefore, a new type of encoder is desirable such that it combines high resolution and low cost.
A sinusoidal electric power steering (EPS) system typically requires an incremental position encoder located on a motor shaft for controlling an electric motor. The encoder typically provides two highresolution quadrature pulse trains EA, EB and an index pulse for determining a rotor position. Upon power-up of the sinusoidal electric power steering (EPS) system, it is generally impossible to determine the rotor position of a sinusoidal electric power steering (EPS) system motor by using the incremental encoder signals until the sinusoidal electric power steering (EPS) system motor is moved across or is encountered by the index pulse position for a first time. Thus, before the sinusoidal electric power steering (EPS) system motor is moved across or is encountered by the index pulse position, the sinusoidal electric power steering (EPS) system motor is typically started by using three additional low-resolution commutation sensors which give initial position estimation, as well as directing currents to the desired motor phases.
The two most popular ways to sense rotary position are based on optical detection and magnetic field variation. Optical encoders are temperature limited and susceptible to dirt. Semiconductor based magnetic sensors (magnetoresistors, or MRs), on the other hand, can work at higher temperature, and is starting to be used in underhood applications.
The encoder typically provides two high-resolution quadrature pulse trains EA, EB represent the two quadrature high-resolution incremental pulse trains. The two high-resolution quadrature pulse trains EA, EB are used to determine the sinusoidal electric power steering (EPS) system motor rotation direction and to increment a sinusoidal electric power steering (EPS) system motor rotor position count. The index signal is a pulse that indicates a zero sinusoidal electric power steering (EPS) system motor rotor position. Generally, a set of three signals, denoted by H1, H2, and H3, are low resolution commutation signals with a 120 electrical degree phase shift, corresponding to a three phase motor winding. Typically, the index pulse is generally aligned with phase A, H1 edges. Therefore, the index pulse is derived from phase A, or H1 edges.
Thus, it can be appreciated that for the sinusoidal electric power steering (EPS) system motor, it is desirous to reduce the cost and size or dimension of the encoder. The above mentioned encoding scheme would need at least one encoding track for the two high-resolution quadrature pulse trains EA, EB. In addition, at least three tracks for low resolution commutation signals, i.e. H1, H2, and H3. Therefore, it is desirable to reduce the size and cost of the encoder by reducing the number of tracks on the encoder.
An encoder for a brushless motor of an Electric Power Steering (EPS) system is disclosed. The encoder includes a commutation track coupled to a shaft for the generation of a commutation signal. The commutation signal in turn is used to generate an index signal every electrical cycle. The encoder further includes a set of high-resolution quadrature signal tracks. The set of high-resolution quadrature signal tracks generate a set of high-resolution signals which are phased at a set angular displacement with respect to one another. A set of sensors sense the commutation signal, and the set of high resolution quadrature signals.